Generally, an image forming apparatus such as copying machine, facsimile, printer or the like transfers an image (e.g., toner image) to a recording medium such as paper sheet, and fixes the image on the recording medium by applying heat to the recording medium to produce an copy print or a record print.
Such image forming apparatus uses a fixing unit to fix the image on the recording medium. In the fixing unit, heat is applied to the recording medium having an unfixed toner image to melt a developing agent and toners included in the unfixed image to fix the toner image on the recording medium.
However, such image forming process may experience degradations on an image to be produced on the recording medium.
For example, the recording medium such as paper has surface irregularities. Because of such surface irregularities, the recording medium and an image carrying member (e.g., photoconductive drum) may not contact closely but have gaps between their surfaces. Such gaps may disturb a transferring-electric field, or may induce Coulomb repulsion between toners. Consequently, such phenomenon may cause degradations on an image to be produced on the recording medium.
In order to cope with such drawbacks, a background art employs a method using an intermediate transfer member driven by a drive roller having a heat source therein, and the intermediate transfer member forms a nip with a pressure member which is pressed to the intermediate transfer member.
In such method, toner images on the intermediate transfer member are heated before the toner images enters the nip, and the heated toner images are fixed on the recording medium at the nip.
Therefore, toner images are transferred from the intermediate transfer member to the recording medium with a heat effect instead of an electrostatic force. Accordingly, the above-mentioned image-quality degradations may less likely to happen on the recording medium.
In order to realize a favorable transferability of toner images from an intermediate transfer member or photoconductive member to a recording medium in such method, the intermediate transfer member or the photoconductive member having a toner image thereon is heated and pressed with a recording medium at first.
Then, the intermediate transfer member or the photoconductive member, the toner image, and the recording medium are contacted and cooled a predetermined time.
And then, the recording medium having the toner image is separated from the intermediate transfer member or photoconductive member.
Such method facilitates a separation of the toner image from the intermediate transfer member or photoconductive member because of such cooling process, thereby a hot-offset of toners can be prevented, wherein the hot-offset is a phenomenon that a part of toners remain on the intermediate transfer member or photoconductive member.
Furthermore, such method can omit a process of applying oily material on the intermediate transfer member or photoconductive member, which is used to facilitate a separation of the toner image from the intermediate transfer member or photoconductive member, thereby such method can favorably realize an oil-less process.
As for a transfer unit and a fixing unit, following background arts can be cited.
One background art uses a fixing belt having an average surface hardness of 0.826 N/mm2 to 2.078 N/mm2, which is measured by a universal hardness testing at an indentation depth of 20 μm.
Another background art uses a fixing belt having another surface hardness expressed with a predetermined formula for universal hardness testing at indentation depths of 4 μm and 20 μm.
Other background arts use an intermediate transfer belt which conducts a transfer and fixing process substantially at the same time
Other background arts also includes a method using a transfer-fixing unit, which has a heater and a heat roller having a movable reflection plate.
Other background arts further includes an image forming apparatus having an intermediate transfer belt, transfer and fixing roller, and a heat source for heating a surface of the fixing roller.
Other background arts further includes a method using a transfer-fixing unit having a heat source for heating a surface of a fixing roller and a reflection plate.
Other background arts further includes a method using a pre-heating unit provided for a heat roller for fixing, an infrared lamp, a reflection mirror, a reflection plate which can adjust its reflection angle and illuminate a face of the recording medium.
The above-mentioned methods used in the background arts conduct a transferring process and a fixing process at the same time for image forming, and such methods can prevent degradations of halftone-image quality in a middle and high concentration range, wherein the degradations in a middle and high concentration range may be caused by a disturbance of toner image or Coulomb repulsion of toners.
However, in a low concentration range, surface irregularities of a recording medium affect on image quality.
For example, toners may not transfer to recessed irregularities on a surface of the recording medium because recessed irregularities may not contact toners.
Accordingly, when the recording medium having a rough surface is used, degradations on images may not be improved in the above-mentioned methods.
As for the middle and high concentration range, when a low-speed operation is conducted for the transfer and fixing process, a favorable image having a uniform glossiness and no-disturbance of pixels may be obtained.
However, when a high-speed operation is conducted for the transfer and fixing process for the middle and high concentration range, transferability of the toner images may degrade.
In such transfer and fixing process, the intermediate transfer member, toner images and the recording medium (e.g., paper) are closely contacted each other and heated, and then the melted toners permeate in the recording medium (e.g., paper), and toner image is fixed on the recording medium (e.g., paper).
However, if the intermediate transfer member has a hard surface, such hard surface may not deform in response to tiny surface irregularities of the recording medium (e.g., paper) when fixing toner images.
Therefore, the intermediate transfer member and the recording medium may not contact closely each other, thereby image-quality degradations such as unevenness of glossiness may happen.
In order to improve quality of images produced by such transferring and fixing process, the intermediate transfer member may need an elastic layer on its outer surface so that the intermediate transfer member can contact closely to the recording medium (e.g., paper) having the toner images.
If the intermediate transfer member does not include an elastic layer on its outer surface, the surface of the intermediate transfer member may not deform in response to tiny surface irregularities on the recording medium (e.g., paper) when fixing the toner images.
IN such a case, the intermediate transfer member and the recording medium cannot contact closely each other, thereby image-quality degradations may happen due to a poor transferability.
Conventionally, in order to reduce the above-mentioned drawbacks, several attempts have been made by paying attention to rubber hardness (e.g., Japan Industrial Standard-A hardness) of the surface of the fixing member.
However, as above-mentioned, image-quality degradations may also happen when a lower pressure is applied to a nip in the fixing process. Therefore, in order to prevent image-quality degradations, it is understand that a higher pressure is required at the nip.
Because a higher pressure may induce a warping of the fixing member, the fixing member may need a core material (e.g., metal) having a relatively higher stiffness, which may be prepared by adjusting a diameter or a thickness of the core material (e.g., metal).
If the diameter or thickness of the core material (e.g., metal) is set a larger value, the core material has a larger heat capacity.
In such a case, the fixing member needs longer time to increase its temperature to a predetermined temperature. Hereinafter, such duration time is referred as “rising-time.”
The “rising-time” of the fixing member can be made shorter by maintaining the temperature of the fixing member at a certain level by pre-heating the fixing member. However, such method is not preferable in view of the energy saving.
On one hand, in order to obtain a higher quality image with a transfer-and-fixing method, the intermediate transfer member and the recording medium should be contacted closely and cooled for a predetermined time after the transfer and fixing process because such cooling process effects a transferability-efficiency of toner images. However, such cooling process may require a larger machine and may increase cost of components.
Furthermore, because the temperature-increased intermediate transfer member should be cooled, a re-heating is required for the intermediate transfer member for a next image forming.
Accordingly, the “rising-time” of the intermediate transfer member becomes longer, and an energy-consumption increases because of such heating-and-cooling cycle.
In case of the high-speed operation, the recording medium travels with a faster speed, thereby a cooling system needs larger components for a fast-cooling, which leads to a larger image forming apparatus and a cost-increase due to an addition of fast-cooling components.
In view of such background, it has been considered that satisfying the following two conditions at the same time is hard to achieve, wherein two conditions are (1) a shorter “rising-time” (i.e., energy saving), and a (2) high quality fixing which can eliminate the effect of the tiny surface irregularities on the surface of the recording medium.